Researchers studied data on exemptions using data from the 18 states and CDC, focusing on school years from 2009-10 to 2016-17.

As a result, hotspots have developed where the risk of kids getting diseases like measles has jumped because of the growing number of non-medical exemptions.

"As larger unvaccinated populations grow, particularly in highly mobile cities, the potential for vaccine-preventable disease outbreaks grows," said Peter Hotez, professor at Baylor College of Medicine and co-editor-in-chief of PLOS Neglected Tropical Diseases; and Melissa Nolan, an assistant professor of epidemiology at the University of South Carolina, in a joint statement.

A 2016 study published by JAMA from researchers at Emory University links refusals of vaccines to increases in cases of measles and whooping cough.

The 12 states showing an increase in exemptions are Arkansas, Arizona, Idaho, Maine, Minnesota, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Texas, and Utah. Six more states — Colorado, Louisiana, Michigan, Missouri, Washington and Wisconsin — also allow the exemptions.

The study also isolated 15 metropolitan areas where more than 400 kindergarten-aged children aren't vaccinated:

Arizona — Phoenix

Utah — Provo, Salt Lake City

Washington — Seattle, Spokane

Oregon — Portland

Michigan — Detroit, Troy, Warren

Texas — Houston, Fort Worth, Plano, Austin

Pennsylvania — Pittsburgh

Missouri — Kansas City

Hotez and Nolan said a second study is underway on these areas to learn how to better target campaigns aimed at encouraging more families to allow vaccinations.

Overall, eight of the top 10 counties with the highest NME rates were in Idaho. The study found the 10 counties with the highest rates "have fewer than 50,000 persons and are located in rural regions of the state."

According to the most recent Centers for Disease Control and Prevention data from 2015, nearly 92% of kids ages 19 to 35 months received a vaccination for measles, mumps and rubella (MMR). Most states require children to have up-to-date vaccinations to enroll in public schools.

"Our concern is that the rising NMEs linked to the anti-vaccine movement in the U.S. will stimulate other countries to follow a similar path," reads an excerpt from the study.